This invention pertains to forward osmosis systems processing of fluids. More particularly, it pertains to forward osmosis systems using hollow fiber membranes and magnetic nanoparticles in a draw solution.
Osmosis is a natural and spontaneous movement of water across a selectively permeable membrane from a region of low solute concentration (pure water) to a region of comparably higher solute concentration such as seawater. The selectivity of the membrane allows for the passage of water while preventing the passage of larger solute molecules and suspended solids. The careful, deliberate selection of the membrane prevents the passage of unwanted molecules. The natural passage of water through the membrane is driven by the difference in the solute concentrations on either side of the membrane. The lower the solute concentration, the greater the driving force of its solvent to permeate the membrane. This driving force is known as osmotic pressure or the net osmotic driving force. There are three types of osmosis. As discussed by Cath et al1 the most familiar is reverse osmosis (RO), which in the field of water treatment, uses hydraulic pressure to oppose, and exceed, the osmotic pressure of an aqueous feed to produce purified water. A second type of osmosis is forward osmosis (FO), which uses the osmotic pressure differential as the driving force for transport across a semi-permeable membrane, which acts as a separator media. A third type of osmosis is pressure retarded osmosis (PRO) that uses osmotic pressure differences between seawater, or concentrated brine, and fresh water to pressurize the saline stream, thereby converting the osmotic pressure of seawater into a hydrostatic pressure that can be used to produce electricity2.
The key basis characteristics of an osmosis system are: the type of osmosis (RO, FO, or PRO); the type and composition of the membrane; and the type and composition of the draw solution. FO is preferred to RO for water purification using several criteria. It generally has greater water recovery, has benign environmental effects, is less subject to environmental fouling, and has lower energy demand. Membranes can either be flat sheet membranes in a plate-and-frame configuration or in a spiral-would configuration; or could be tubular. In turn, tubular membranes can be either tubes or hollow fibers. Cath et al1 discuss the advantages of hollow fiber membranes. They point out that hollow fiber membranes can support high hydraulic pressure without deforming and can be easily packed in bundles directly within a holding vessel. They are also relatively easy to fabricate in modular form. Also, they allow liquids to flow freely on the feed side of the membrane. Other advantages of hollow fiber membranes are they are much cheaper to manufacture and they can have several hundred times the surface area per unit volume than flat sheet spiral wound membranes.
A variety of compositions can be used for the draw solution. In an early commercial application of FO, Wickenden in U.S. Pat. No. 2,116,920 teaches the use of calcium chloride as a draw solution in the concentration of fruit juices. In another early patent, Batchelder in U.S. Pat. No. 3,171,799 teaches the use of a volatile solute, such as sulfur dioxide, in a draw solution for the demineralization of water. Recently interest in draw solutions has centered on those containing magnetic nanoparticles. Magnetic particles in the draw solution have the advantage of being able to be readily separated from the product water of a purification or desalination process with use of magnetic fields. They can also be readily recycled back into the draw solution. A kind of nanoparticles that is currently of interest is a material referred to as Magnetoferritins. As Oriard et al describe in US 2007/0278153, it is magnetite bound to a protein such as ferritin wherein the magnetite is the core and the protein is the spherical cover. The use of magnetite nanoparticles is also taught by Etemad et al in US 2010/0051557 in the context of removing heavy metals from aqueous media by means of adsorption and magnetic capturing. Etemad et al mention that the magnetite is superparamagnetic but does not indicate that they are coated with a protein. Superparamagnetic iron oxide nanoparticles (SPIONs) are also the subject of intense research for various biomedical applications as described by Latorre et al3.